This invention relates to an improved procedure for regeneration of spent sulfuric acids, especially those recovered from petrochemical and refinery processes. More particularly, the invention relates to a novel process for regenerating spent sulfuric acid in which a pre-drying system is employed to improve product acid strength.
In one known process for the regeneration of spent (waste) sulfuric acids, the acids are fed to a high temperature (e.g., 1600.degree. F. to about 2300.degree. F.) furnace where they are transformed into sulfur dioxide, water vapor and other gaseous products. The sulfur dioxide and gaseous products are normally passed to a waste heat boiler to remove heat, then to a scrubbing zone where they are contacted with dilute sulfuric acid, cooled to a temperature of from about 150.degree. F. to about 260.degree. F., and become humid on cooling. The gases are then cooled further in a heat exchanger (preferably water cooled) to a temperature of from about 100.degree. F. to about 120.degree. F. Appreciable condensation occurs in this exchanger, and the water is removed, a portion of the water being returned to the scrubber. The gases pass to an acid mist removal zone where acid mist is collected and removed. The gases are then forwarded to a drying zone or tower. Prior to the entry of the gases into the drying tower, atmospheric air is added to bring the oxygen content to a value suitable for converter operation. In the drying tower, water remaining in the gases is removed by contacting the gases with strong sulfuric acid (93 percent to 98 percent by weight H.sub.2 SO.sub.4). Any water remaining at this point appears in the product acid.
One problem associated with this procedure is that when refinery or petrochemical sulfuric acids of high hydrocarbon or water content are regenerated, the relatively low sulfuric acid content (e.g., 25 weight percent to 92 weight percent) results in gas streams containing less than desirable amounts of sulfur dioxide. Consequently, the amount of water collected in the drying zone or tower may be such that sulfuric acid of the desired strength cannot be produced. This problem is exacerbated by the increased efficiency in utilization of sulfuric acid by refinery and petrochemical users and has resulted in a decrease in the total amount of acid recoverable from waste streams.